The hoop uniformly charged with a positive charge rests on four rollers and can rotate. One section of the hoop passes through a hole made in parallel differently charged plates. According to the inventor, the area of the hoop located between the plates will be attracted to the negative plate and repelled from the positive one. There is no field outside the plates. Thus, the rotation of the hoop will be maintained even if there is resistance to movement — a perpetual motion machine is obtained. What is the inventor's mistake? Prove that the moment of forces acting on such a hoop in any electrostatic field is zero.